1. Field of the Invention
This invention relates generally to polyurethane and/or urea elastomers and, more particularly, to reaction molded polyurethane elastomers and the like. Specifically, the present invention relates to a novel molded elastomer having both high and low temperature resistance in dynamic applications.
2. Description of the Prior Art
Polyurethane based elastomers are often prepared by reacting a relatively high equivalent weight active hydrogen-containing material, such as a polyol, and a relatively low equivalent weight active hydrogen-containing material, such as a chain extender, with a polyisocyanate. In preparing the elastomer, the reactive components and any catalysts or other optional additives are blended together and then transferred to a mold of suitable shape where the formulation is cured. Traditionally, this blending of the components and other additives has been performed in a batch process. It is typical practice to cure the elastomer in the mold, particularly in a batch process, until it is capable of maintaining the molded shape, and then demolding the elastomer and postcuring it until the polymerization is complete. In this manner, the molds may be used more often thereby permitting higher production rates.
Since it is usually desirable to produce as many molded parts as possible in a given period of time, it is important that the residence time in the mold be as short as possible. Accordingly, it is desirable that the elastomer formulation cure relatively rapidly in the mold to a state at which the elastomer can be demolded and postcured. In batch processing, however, it is necessary that the formulation not cure too quickly since some time is required to blend the batch components of the formulation and then transfer the blend to the mold.
In practice, particularly with conventional casting techniques and batch processing using deactivated amine chain extenders, no catalyst is used. In such instances, typical in-mold residence times range from about 1-5 hours, depending on the particular components used and the cure temperature. In addition, certain hand/machine cast techniques which are heavily catalyzed form in well under one hour mold time. Such cast techniques have been particularly useful in forming roller skate wheels and the like. However, in order to decrease energy usage and increase productivity, it is desirable to provide an elastomer formulation which cures much more quickly and at a lower mold temperature. A common target for preparing elastomers has been a thirty minute residence time at about 100.degree.-130.degree. C. utilizing a catalyst system. However, it has at times been difficult to obtain optimal properties and morphology from elastomers prepared utilizing catalyst and batch processing systems.
Reaction Injection Molding (RIM) is a technique for the rapid mixing and molding of large, fast curing urethane parts. RIM polyurethane parts have traditionally been used in a variety of exterior body applications on automobiles where their light weight contributes to energy conservation. RIM parts are generally made by rapidly mixing active hydrogen containing materials with polyisocyanate and simultaneously injecting the mixture into a mold where reaction proceeds. These active hydrogen containing materials typically include a high molecular weight polyhydric polyether and/or a low molecular weight active hydrogen containing compound, i.e., a chain extender. Moreover, RIM parts for automotive application typically are reacted very quickly and demolded in 1-2 minutes. After reaction and demolding, the parts may be subjected to an additional curing step by placing the parts at an ambient temperature of about 250.degree. F. or greater for 4-24 hours. Unfortunately, the extremely rapid reaction time causes a loss of control over the morphological structure.
Typical of accepted RIM practice is to place all components except the isocyanate in one vessel (called the B component) and the isocyanate in another vessel (called the A component) prior to reaction and then mixing the A and B components together in a selected stoichiometric balance in a mold. U.S. Pat. No. 4,297,444 discloses a modification to this traditional procedure. This modification includes the prereacting of a portion of the high molecular weight polyhydric polyether with a portion of the isocyanate. The chain extender and the remaining polyhydric polyether, if any, is then mixed together along with the prepolymer in a RIM process to react the components to form a RIM polyurethane elastomer.
Numerous other patents disclose RIM elastomers and their preparation including U.S. Pat. Nos. 4,806,615, 4,742,090, 4,404,353, 4,732,919, 4,530,941 and 4,607,090.
As indicated above, RIM elastomers have been readily utilized as automobile fascia and other components thereof, such as fenders, steering wheels, dashboards, and various other structural and flexible components. The significant advantage in the RIM processing technique is that mixing, reaction and molding injection all take place simultaneously so as to reduce the amount of residence time in the mold. Thus, RIM elastomers have found wide acceptance in a variety of consumer and industrial applications.
However, the majority of these product applications involve static and/or nonloaded applications, wherein the component part is not subjected to external loading. Another area of concern relates to temperature resistance wherein certain products are subjected to temperature extremes, both elevated and lowered temperatures. In certain applications, such as automobile timing and power transmission belts, V-belts, micro-ribbed belts, molded boots for U-joints, and the like, the product is subjected to both high and low temperatures in dynamic loading conditions. In such situations, elastomers have to date been unacceptable for long term usage due to their tendency to yield and/or crack under dynamic loading at both high and low temperatures. Thus, there remains a need for polyurethane elastomers that have the characteristics necessary to withstand dynamic loading under high and low temperature conditions.